The most efficient fossil fuel-based engines are massive two-stroke diesels found on extremely large ships and in some peaker power gensets which currently have efficiencies just over 51% and BSFC's over 275.
https://en.wikipedia.org/wiki/Brake_specific_fuel_consumptio...
In this case, he may have matched the performance of very large engines, which have the advantages of optimizing for scale, but time will tell whether this solution can deliver.